This invention relates generally to semiconductor laser diodes and, more particularly, to techniques for laterally confining the optical output of semiconductor laser diodes.
High-power laser light sources are required for a variety of optical systems, such as optoelectronic logic circuits, fiber-optic communication systems, and for optically pumping solid state lasers. Semiconductor laser diodes are particularly well suited as laser light sources for these optical systems because of their small size, high power efficiency, reliability, direct current modulation and operation at wavelengths having low transmission and dispersion losses in glass fiber optics.
One particular application for high-power semiconductor laser diodes is an optical pump source for erbium-doped optical amplifiers. These amplifiers are used in long-range fiber-optic communication systems and employ an erbium-doped fiber that is pumped at an absorption wavelength of 0.98 microns (.mu.m). The erbium-doped fiber emits light at a wavelength of 1.55 microns, which has very low losses in silica-based optical fibers. However, optimal coupling between the semiconductor laser diode and the single-mode fiber requires that the laser diode operate at its fundamental transverse and lateral modes to produce a single-mode laser beam with an aperture spot size closely matching the 4 to 8 micron diameter of the optical fiber.
By way of general background, a semiconductor laser is a diode device in which a forward bias voltage is applied across an active layer and a pair of cladding layers. One cladding layer is an n-doped layer and the other is a p-doped layer so that excess electrons from the n-doped layer and excess holes from the p-doped layer are injected into the active layer by the bias voltage, where they recombine. At current levels above a threshold value, stimulated emission occurs and a monochromatic, highly-directional beam of light is emitted from the active layer. A resonant cavity is formed in the active layer at either end of the device by a highly-reflective surface and a partially-reflective surface through which the beam emerges. The cladding layers usually have a lower index of refraction than the active layer to provide a dielectric waveguide that transversely confines the laser light to the active layer. Active layers less than approximately 0.3 microns usually provide a dielectric waveguide in which only the fundamental transverse mode is supported.
Various techniques are typically employed to laterally confine the optical output of a semiconductor laser diode for operation at the fundamental lateral mode. One technique is gain guiding, which utilizes a narrow electrical contact to supply current to the device. The narrow electrical contact limits lasing in the active layer to the narrow region of the electrical contact. However, at high power levels, gain-guided laser diodes have strong instabilities and generate broad, highly-astigmatic double-peaked beams.
Another technique for laterally confining the optical output of a semiconductor laser diode is index guiding. Index guiding employs various dielectric waveguide structures to laterally confine the laser light. These waveguide structures are either positive-index guides, in which the index of refraction is higher in the region aligned with the laser element and lower in the regions surrounding the laser element, or negative-index guides, in which the index of refraction is lower in the region aligned with the laser element and higher in the regions surrounding the laser element. Positive-index guiding effectively traps light in the laser element, while negative-index guiding, or antiguiding, allows light to leak out of the lasing element. However, conventional index-guided laser diodes have aperture spot sizes on the order of 2 to 3 .mu.m and operate at single-mode power levels of less than 200 mW, which is inadequate for pumping optical amplifiers for long range transmission. Accordingly, there has been a need for a semiconductor laser diode having an aperture spot size on the order of 4 to 8 microns that operates at higher power levels, preferably greater than 250 mW. The present invention clearly fulfills this need.